The present invention relates apparatus for controlling the operation of an electric motor; and particularly to such apparatus which protect the motor against over heating.
Today, most industrial motors of one horsepower or larger are three-phase induction motors. These devices typically run very close to their thermal limits in order to achieve higher specific outputs. Thus, the user requires a finer degree of protection to avoid unnecessary Shutdowns and production loss. Adequate protection of these motors can be achieved through systems designed to detect overload, short-circuits, and phase-to-phase faults.
Overloading is the most common cause of motor failure but only if the overloading is sustained. A sustained overload condition can cause serious and continuous temperature build-up within the motor winding which shortens the life of insulation and can ultimately result in complete burn-out. Conventional overload protection has involved the use of electromagnetic overload relays or thermal overload relays. These are current operated devices which have inverse characteristics that may differ considerably from the thermal characteristics of the motor and its surroundings. To be an effective safeguard, prediction devices should not only match the heating characteristics of the motor, but also its cooling characteristics. These desirable characteristics are more nearly met in thermal overload relays, but unfortunately the heating and cooling characteristics of many motor designs are so diverse as to make it nearly impossible to provide a single thermal relay to match them.
Electronic motor protection devices have more recently been introduced. These devices use electrical resistance-capacitance (RC) analog to simulate the heating conditions of a motor. In the RC thermal model, heat flowing into the motor from conductor and core losses is represented by a current source in the equivalent circuit. The resistance R represents the motor's thermal resistance and the capacitance C represents the motor's thermal capacitance. Digital circuits and microprocessors have been employed to perform the protection function in a manner similar to the previous analog devices and in addition provide a range of other protection functions. Electronic protection devices are designed to be applicable to a wide range of machine ratings, and facilities and provided to match the protection characteristics to the installed motor. Such conventional overload protection devices assume effective ventilation of the motor. If an air inlet becomes blocked through non-maintenance, the motor may overheat even though the voltage and current levels applied to the motor are normal. In addition, when the motor and its protection device are situated in locations with different ambient temperatures, the protection device may fail to register overheating that results from high ambient temperature at the motor.
It is generally argued that overload protection should take the form of devices that provide a direct indication of winding temperature and which ignore current loading. The basis of this argument is that full use can then be made of an inherent overload capacity a motor may have; particularly under cyclic conditions, but with the knowledge that the motor is protected against overheating and possible burn-out. This argument can be resolved by installing temperature detectors, such as thermistors, in the motor's winding during manufacture. This method is known to be costly and involves additional wiring and installation expense.
A number of indirect techniques have been developed to measure the temperature of motor windings while the motor is in operation. For example, U.S. Pat. No. 1,728,830 discloses a temperature indicating device for a dynamo-electric machine which comprises a plurality of resistors connected to the field-producing winding of the machine so that a Wheatstone bridge circuit is formed. An ammeter is connected to the galvanometer points on the bridge in order that the thermal condition of the winding can be ascertained by the change of resistance of the field producing winding.
U.S. Pat. No. 4,083,001 places an asymmetric resistance in series with the motor circuit. In particular, the asymmetric resistance device comprises two series diodes in parallel with a third diode in the opposite direction. The resistance of the motor winding is then determined from measurements of the direct current component and the corresponding voltage using a magnetic amplifier with a bias winding excited in response to the voltage of the asymmetric resistance and a control winding excited by the motor current. An indication is obtained when the current flows below a level that indicates a resistance corresponding to an overload temperature condition.
With the advent of microelectronic circuitry, microprocessor based control circuits, such as the one disclosed in U.S. Pat. No. 4,996,470, have been developed to start and stop a motor. Specifically, the microprocessor controls the times at which thyristor switches are triggered to vary the amount of time during each cycle of the alternating supply current that the switches are conductive. This varies the amount of electricity applied to the motor. The zero crossings of the a.c. supply voltage for each phase is detected and the corresponding thyristor switch is triggered after a defined delay period. The amount of delay at which the thyristors are triggered is varied to control the magnitude of the voltage applied to the motor. For example, when the motor is started, a relatively long delay can be utilized which is then gradually decreased to apply progressively more electricity and increase the motor speed in a controlled fashion. The inverse operation can be employed to downwardly ramp the speed of the motor to stop the motor in a controlled fashion. Since these types of motor controllers already provide sophisticated computational capacity, it is desirable to incorporate overload protection into such controllers.